Support tsconfig paths without baseurl (#34926)

tsconfig behaves differently with `tsc` vs. Next.js. When `baseurl` is omitted, `paths` should be resolved against the tsconfig location. Consequentially, in this case the paths must start with `./`. This PR aligns Next.js behavior with `tsc` around `paths` without `baseurl`. 

Related: microsoft/TypeScript#40101

packages/next/build/load-jsconfig.ts

@@ -49,6 +49,7 @@ export default async function loadJsConfig(
     typeScriptPath && (await fileExists(tsConfigPath))
   )
 
+  let implicitBaseurl
   let jsConfig
   // jsconfig is a subset of tsconfig
   if (useTypeScript) {
@@ -65,17 +66,24 @@ export default async function loadJsConfig(
     )) as typeof import('typescript')
     const tsConfig = await getTypeScriptConfiguration(ts, tsConfigPath, true)
     jsConfig = { compilerOptions: tsConfig.options }
+    implicitBaseurl = path.dirname(tsConfigPath)
   }
 
   const jsConfigPath = path.join(dir, 'jsconfig.json')
   if (!useTypeScript && (await fileExists(jsConfigPath))) {
     jsConfig = parseJsonFile(jsConfigPath)
+    implicitBaseurl = path.dirname(jsConfigPath)
   }
 
   let resolvedBaseUrl
-  if (jsConfig?.compilerOptions?.baseUrl) {
-    resolvedBaseUrl = path.resolve(dir, jsConfig.compilerOptions.baseUrl)
+  if (jsConfig) {
+    if (jsConfig.compilerOptions?.baseUrl) {
+      resolvedBaseUrl = path.resolve(dir, jsConfig.compilerOptions.baseUrl)
+    } else {
+      resolvedBaseUrl = implicitBaseurl
+    }
   }
+
   return {
     useTypeScript,
     jsConfig,

test/integration/jsconfig-paths/test/index.test.js

@@ -3,14 +3,17 @@
 import fs from 'fs-extra'
 import { join } from 'path'
 import cheerio from 'cheerio'
+import * as path from 'path'
 import {
   renderViaHTTP,
   findPort,
   launchApp,
   nextBuild,
   killApp,
   check,
+  File,
 } from 'next-test-utils'
+import * as JSON5 from 'json5'
 
 const appDir = join(__dirname, '..')
 let appPort
@@ -21,7 +24,7 @@ async function get$(path, query) {
   return cheerio.load(html)
 }
 
-describe('TypeScript Features', () => {
+function runTests() {
   describe('default behavior', () => {
     let output = ''
 
@@ -141,4 +144,30 @@ describe('TypeScript Features', () => {
       ).toBe(false)
     })
   })
+}
+
+describe('jsconfig paths', () => {
+  runTests()
+})
+
+const jsconfig = new File(path.resolve(__dirname, '../jsconfig.json'))
+
+describe('jsconfig paths without baseurl', () => {
+  beforeAll(() => {
+    const jsconfigContent = JSON5.parse(jsconfig.originalContent)
+    delete jsconfigContent.compilerOptions.baseUrl
+    jsconfigContent.compilerOptions.paths = {
+      '@c/*': ['./components/*'],
+      '@lib/*': ['./lib/a/*', './lib/b/*'],
+      '@mycomponent': ['./components/hello.js'],
+      '*': ['./node_modules/*'],
+    }
+    jsconfig.write(JSON.stringify(jsconfigContent, null, 2))
+  })
+
+  afterAll(() => {
+    jsconfig.restore()
+  })
+
+  runTests()
 })

test/integration/typescript-paths/test/index.test.js

@@ -2,7 +2,15 @@
 
 import { join } from 'path'
 import cheerio from 'cheerio'
-import { renderViaHTTP, findPort, launchApp, killApp } from 'next-test-utils'
+import * as path from 'path'
+import {
+  renderViaHTTP,
+  findPort,
+  launchApp,
+  killApp,
+  File,
+} from 'next-test-utils'
+import * as JSON5 from 'json5'
 
 const appDir = join(__dirname, '..')
 let appPort
@@ -13,7 +21,7 @@ async function get$(path, query) {
   return cheerio.load(html)
 }
 
-describe('TypeScript Features', () => {
+function runTests() {
   describe('default behavior', () => {
     beforeAll(async () => {
       appPort = await findPort()
@@ -46,4 +54,34 @@ describe('TypeScript Features', () => {
       expect($('body').text()).toMatch(/Not aliased to d\.ts file/)
     })
   })
+}
+
+describe('typescript paths', () => {
+  runTests()
+})
+
+const tsconfig = new File(path.resolve(__dirname, '../tsconfig.json'))
+
+describe('typescript paths without baseurl', () => {
+  beforeAll(async () => {
+    const tsconfigContent = JSON5.parse(tsconfig.originalContent)
+    delete tsconfigContent.compilerOptions.baseUrl
+    tsconfigContent.compilerOptions.paths = {
+      'isomorphic-unfetch': ['./types/unfetch.d.ts'],
+      '@c/*': ['./components/*'],
+      '@lib/*': ['./lib/a/*', './lib/b/*'],
+      '@mycomponent': ['./components/hello.tsx'],
+      'd-ts-alias': [
+        './components/alias-to-d-ts.d.ts',
+        './components/alias-to-d-ts.tsx',
+      ],
+    }
+    tsconfig.write(JSON.stringify(tsconfigContent, null, 2))
+  })
+
+  afterAll(() => {
+    tsconfig.restore()
+  })
+
+  runTests()
 })